1. Field of the Invention
The present invention relates to a light scanning device and an image forming apparatus using the same and, more particularly, to a light scanning device which is suitable for a laser beam printer or digital copying apparatus which has, for example, an electrophotographic process of reflecting/deflecting (deflecting/scanning) a light beam, optically modulated and emitted from a light source means, by using a deflecting element formed from a rotating polyhedral mirror or the like, and recording image information by optically scanning a surface to be scanned through an imaging optical system having f-xcex8 characteristics.
2. Description of Related Art
In a conventional scanning optical device for a laser beam printer (LBP) or the like, the light beam which is optically modulated in accordance with an image signal and emitted from a light source means is cyclically deflected by a light deflector formed from, e.g., a rotating polyhedral mirror (polygon mirror), and the light beam is focused into a spot on the surface of a photosensitive recording medium (photosensitive drum) and optically scanned on the surface by an imaging optical system having f-xcex8 characteristics.
FIG. 17 is a schematic view showing the main part of a conventional light scanning device.
Referring to FIG. 17, the divergent light beam emitted from a light source means 1 is converted into a substantially parallel light beam by a collimator lens 2, and the light beam is limited by a stop 3 to be incident on a cylindrical lens 4 having predetermined refracting power only in the sub scanning direction. Of the substantially parallel light beam incident on the cylindrical lens 4, the light in a main scanning cross-section emerges without any change. The light in a sub-scanning cross-section is focused and substantially formed into an almost line image on a deflecting surface (reflecting surface) 5a of a light deflector 5.
The light beam reflected/deflected by the deflecting surface 5a of the light deflector 5 is guided onto the photosensitive drum surface 8 serving as a surface to be scanned through an imaging optical system (f-xcex8 lens system) 6 having f-xcex8 characteristics. By rotating the light deflector 5 in the direction indicated by an arrow A, the photosensitive drum surface 8 is optically scanned to record image information.
In such a light scanning device, in order to record high-resolution image information, for example, the following requirements need to be satisfied. The curvature of the image surface is properly corrected throughout the surface to be scanned. Distortion characteristics (f-xcex8 characteristics) with uniform velocity characteristics are set between a scanning angle xcex8 and an image height Y. A spot diameter on the image surface at each image height is uniform. Conventionally, various types of light scanning devices that satisfy such optical characteristics or correction optical systems (f-xcex8 lenses) have been proposed.
As laser beam printers and digital copying apparatuses have decreased in size and cost, similar requirements are imposed on light scanning devices.
As an arrangement that satisfies such requirements, a light scanning device having an f-xcex8 lens formed from a single lens is disclosed in, for example, Japanese Patent Application Laid-Open Nos. 4-50908 and 9-33850.
According to Japanese Patent Application Laid-Open No. 4-50908, a high-order aspherical surface is used in the main scanning direction of an f-xcex8 lens to relatively properly correct aberration characteristics. However, since the magnification in the sub scanning direction between a light deflector and a surface to be scanned is not uniform, the spot diameter in the sub scanning direction tends to change with a change in image height.
According to Japanese Patent Application Laid-Open No. 9-33850, the curvatures of at least two of the lens surfaces of an f-xcex8 lens in the light scanning device in the sub scanning direction continuously change at the effective portion of an imaging lens along the main scanning direction independently of the curvature in the main scanning direction. This makes it possible to control the position of the principal plane in the sub scanning direction by bending the two surfaces and make the sub scanning magnification at each image height uniform, thereby making the spot diameter uniform.
In the above proposal, in order to make the sub scanning magnification uniform, the position of the principal plane is so controlled as to make the main scanning uniform by bending at least two surfaces. Although this allows completely independent setting of a main scanning shape and sub scanning shape, requirements such as suppression of an increase in lens thickness tend to make the lens shape in the main scanning direction have a relatively large aspherical surface amount.
A lens having a large aspherical surface amount in the main scanning direction like the one described above is subjected to considerable deteriorations in optical performance due to the arrangement errors of the respective lens surfaces and lens. Of the deteriorations in optical performance, a scanning line bend in the sub scanning direction, in particular, cannot be corrected by adjusting mirrors and the like arranged in the device body, unlike scanning line height deviation, scanning line inclination, and the like. This therefore poses a serious problem. In order to suppress a scanning line bend to a low level, the respective lens surfaces and the lens need to be arranged with high precision in accordance with design values or an adjusting mechanism needs to be provided for the lens to adjust the arrangement according to the design values.
In a color image forming apparatus in which light scanning devices are respectively arranged in correspondence with four photosensitive devices (photosensitive drums), and latent images are formed on them by laser beams to form Y (yellow), M (magenta), C (cyan), and Bk (black) original images on the corresponding photosensitive device surfaces, since the four color images, i.e., the Y, M, C, and Bk images, formed on the respective photosensitive device surfaces are superimposed on a transfer medium such as a paper sheet, if scanning lines of the light scanning devices corresponding to the respective photosensitive devices bend, errors are produced in the shapes of the scanning lines among four colors. This causes color misregistration on the image on the transfer medium, and hence results in a considerable deterioration in image quality.
FIG. 18 shows a scanning line moving amount on the surface to be scanned when a scanning optical element and each optical element surface are decentered (by 50 xcexcm) in a direction (sub scanning direction) perpendicular to the main scanning direction in an embodiment disclosed in Japanese Patent Application Laid-Open No. 9-33850. As is obvious from FIG. 18, when an optical surface is decentered, a large scanning line bend occurs, and an improvement in arrangement precision or decentering adjustment is required to obtain a high-quality image.
Referring to FIG. 18, the R1 curve represents a scanning line bend amount when only the incident surface of an f-xcex8 lens 6 is decentered in the sub scanning direction; the R2 curve, a scanning line bend amount when only the exit surface of the f-xcex8 lens 6 is decentered; and the block curve, a scanning line bend amount when both the incident and exit surfaces of the f-xcex8 lens 6 are decentered.
The present invention has been made to solve the above problems, and has as its object to provide a light scanning device which has an f-xcex8 lens formed from a single lens, and can suppress a scanning line bend caused by the arrangement error of the lens to a low level by optimally shaping the f-xcex8 lens, and an image forming apparatus using the device.
An object of the present invention is to provide a light scanning device having an imaging optical system for guiding a light beam emitted from light source means to a deflecting element, and forming the light beam deflected by the deflecting element into an image on a surface to be scanned, wherein the imaging optical system is formed from a single lens, a cross-sectional shape of an exit surface of the single lens in a main scanning direction is an arc shape, power of the exit surface in a sub scanning direction satisfies 0.9xe2x89xa6xcfx86s2/xcfx86sxe2x89xa61.1 where xcfx86s is power of an overall imaging optical system in the sub scanning direction and xcfx86s2 is power of the exit surface in the sub scanning direction, and the arc shape in the main scanning direction satisfies       0.9    xc3x97                  L                  b          ⁢                      xe2x80x83                    ⁢          o                            L                  a          ⁢                      xe2x80x83                    ⁢          o                      ≤            L              b        ⁢                  xe2x80x83                ⁢        θ                    L              a        ⁢                  xe2x80x83                ⁢        θ              ≤      1.1    xc3x97                  L                  b          ⁢                      xe2x80x83                    ⁢          o                            L                  a          ⁢                      xe2x80x83                    ⁢          o                    
where Lao is an equivalent air distance from the deflecting means on an optical axis to the exit surface of the single lens, Lbo is a distance from the exit surface of the single lens to the surface to be scanned, Laxcex8 is an equivalent air distance from the deflecting means at an off-axis position to the exit surface of the single lens, and Lbxcex8 is a distance from the exit surface of the single lens to the surface to be scanned.
In the present invention, it is preferable that the power of the exit surface in the sub scanning direction or/and the power of the incident surface in the sub scanning direction changes without correlating to a shape in the main scanning direction.
In the present invention, it is preferable that a radius of curvature of the exit surface in the sub scanning direction changes from an on-axis position to an off-axis position.
In the present invention, it is preferable that the incident and exit surfaces are anamorphic surfaces.
In the present invention, it is preferable that the shape of the incident surface in the main scanning direction is aspherical.
In the present invention, it is preferable that letting L be an optical path length of the imaging optical system, and f be a focal length, the optical path length and focal length satisfy 1.35fxe2x89xa6Lxe2x89xa61.55f.
In the present invention, it is preferable that, of the lens shape of the imaging optical system in the main scanning direction, there is no inflection point in a curvature change on a surface which uses an aspherical surface.
In the present invention, it is preferable that the imaging optical system is manufactured by plastic molding.
In the present invention, it is preferable that a multi-beam laser is used as a light source of the imaging optical system.
In the present invention, it is preferable that the power of the exit surface in the sub scanning direction satisfies 0.95xe2x89xa6xcfx86s2/xcfx86sxe2x89xa61.05 where xcfx86s is power of the overall imaging optical system in the sub scanning direction and xcfx86s2 is power of the exit surface in the sub scanning direction.
In the present invention, it is preferable that the arc shape in the main scanning direction satisfies       0.95    xc3x97                  L                  b          ⁢                      xe2x80x83                    ⁢          o                            L                  a          ⁢                      xe2x80x83                    ⁢          o                      ≤            L              b        ⁢                  xe2x80x83                ⁢        θ                    L              a        ⁢                  xe2x80x83                ⁢        θ              ≤      1.05    xc3x97                  L                  b          ⁢                      xe2x80x83                    ⁢          o                            L                  a          ⁢                      xe2x80x83                    ⁢          o                    
where Lao is the equivalent air distance from the deflecting means on the optical axis to the exit surface of the single lens, Lbo is the distance from the exit surface of the single lens to the surface to be scanned, Laxcex8 is the equivalent air distance from the deflecting means at an off-axis position to the exit surface of the single lens, and Lbxcex8 is the distance from the exit surface of the single lens to the surface to be scanned.
In the present invention, it is preferable that the shape of the exit surface of the single lens is an arc shape.
In the present invention, it is preferable that the shape of the incident surface of the single lens within a sub scanning cross-section is flat.
Further object of the present invention is to provide an image forming apparatus comprising above-described light scanning device, a photosensitive device placed on the surface to be scanned, a developing device for developing an electrostatic latent image formed on the photosensitive device by a light beam scanned by the light scanning device as a toner image, a transferring device for transferring the developed toner image onto a transfer medium, and a fixing device for fixing the transferred toner image on the transfer medium.
Further object of the present invention is to provide an image forming apparatus comprising above-described light scanning device, and a printer controller for converting code data input from an external device into an image signal, and inputting the signal to the light scanning device.
Further object of the present invention is to provide an image forming apparatus comprising a plurality of above-described imaging optical systems, each of the imaging optical systems recording image information on photosensitive devices corresponding to each color or a plurality of colors.
Further object of the present invention is to provide a light scanning device having an imaging optical system for guiding a light beam emitted from light source means to a deflecting element, and forming the light beam deflected by the deflecting element into an image on a surface to be scanned, wherein the imaging optical system is formed from a single lens formed by a molding process, a cross-sectional shape of an exit surface of the single lens in a main scanning direction is an arc shape, power of the exit surface in a sub scanning direction satisfies 0.9xe2x89xa6xcfx86s2/xcfx86sxe2x89xa61.1 where xcfx86s is power of the overall imaging optical system in the sub scanning direction and xcfx86s2 is power of the exit surface in the sub scanning direction, and the arc shape in the main scanning direction satisfies       0.9    xc3x97                  L                  b          ⁢                      xe2x80x83                    ⁢          o                            L                  a          ⁢                      xe2x80x83                    ⁢          o                      ≤            L              b        ⁢                  xe2x80x83                ⁢        θ                    L              a        ⁢                  xe2x80x83                ⁢        θ              ≤      1.1    xc3x97                  L                  b          ⁢                      xe2x80x83                    ⁢          o                            L                  a          ⁢                      xe2x80x83                    ⁢          o                    
where Lao is an equivalent air distance from the deflecting means on an optical axis to the exit surface of the single lens, Lbo is a distance from the exit surface of the single lens to the surface to be scanned, Laxcex8 is an equivalent air distance from the deflecting means at an off-axis position to the exit surface of the single lens, and Lbxcex8 is a distance from the exit surface of the single lens to the surface to be scanned.